Growth and characterization of GaAs layers on polished Ge/Si by selective aspect ratio trapping

Epitaxial GaAs layers have been deposited on polished Ge film grown on exactly (0 0 1) oriented Si substrate by metal-organic chemical vapor deposition (MOCVD) via aspect ratio trapping (ART) method. Double-crystal X-ray diffraction shows that the full-width at half-maximum (FWHM) of the (4 0 0) reflection obtained from 1 μm GaAs is 140 arcsec. Scanning electron microscopy (SEM) of the GaAs layer surface shows that the amount of antiphase domain defects (APD) raised from GaAs/Ge interface using Ge ART on Si is dramatically reduced compared to GaAs layers grown on exact (0 0 1) Ge substrate. Defect reduction and Ge diffusion at vicinal GaAs/Ge interface were investigated via cross-section transmission electron microscopy (X-TEM) and secondary ion mass spectrometry (SIMS). Film morphology and optical properties were evaluated via SEM and room temperature photoluminescence (PL).     

Properties of self-assembled InAs quantum dots grown by various growth techniques

The properties of self-assembled InAs quantum dots (QDs) grown by molecular beam epitaxy on GaAs substrates were investigated. The surface properties of samples were monitored by reflection high-energy electron diffraction to determine growth. Photoluminescence (PL) and transmission electron microscope (TEM) were then used to observe optical properties and the shapes of the InAs-QDs. Attempts were made to grow InAs-QDs using a variety of growth techniques, including insertion of the InGaAs strained-reducing layer (SRL) and the interruption of In flux during QD growth. The emission wavelength of InAs-QDs embedded in a pure GaAs matrix without interruption of In flux was about 1.21 μm and the aspect ratio was about 0.21. By the insertion InGaAs SRL and interruption of In flux, the emission wavelength of InAs-QDs was red shifted to 1.37 μm and the aspect ratio was 0.37. From the PL and TEM analysis, the properties of QDs were improved, particularly when interruption techniques were used.     

Self-assembled InAs quantum dots with two different matrix materials

The effects of matrix materials on the structural and optical properties of self-assembled InAs quantum dots (QDs) grown by a molecular beam epitaxy were investigated by atomic force microscopy, cross-sectional transmission electron microscopy (TEM), and photoluminescence (PL) spectroscopy. Cross-sectional TEM image indicated that the average lateral size and height of InAs QDs in a GaAs matrix on a GaAs substrate were 20.5 and 5.0 nm, respectively, which showed the PL peak position of 1.19 μm at room temperature. The average lateral size and height of InAs QDs buried in an InAlGaAs matrix on InP were 26.5 and 3.0 nm, respectively. The PL peak position for InP-based InAs QDs was around 1.55 μm at room temperature. If we only consider the size quantization effects, the difference in PL peak position between two QD systems with different matrices may be too large. The large difference in peak position can be mainly related to the QD size as well as the strain between the QDs and the matrix materials. The intermixing between the QDs and the matrix materials can partially change the In composition of QDs, resulting in the modification of the optical properties.     

Study of Ge epitaxial growth on Si substrates by cluster beam deposition

Ge epitaxial layers with reasonable quality were grown on Si (1 1 1) substrates by cluster beam deposition (CBD) process. Molecular dynamics study of the low energy Ge clusters deposition process utilizing the Stillinger–Weber two- and three-body interaction potentials was carried out to compare the experimental results. Both experimental and simulation results prove that the substrate temperature plays a dominant role in the epitaxial growth of Ge films in CBD process. The influence mechanisms of temperature are discussed.     

Self-alignment of self-assembled InAs quantum dots

The lateral self-alignment properties of self-assembled InAs quantum dots (QDs) on a conventional GaAs (1 0 0) substrate by molecular beam epitaxy were investigated. The shape and optical properties of QDs were investigated by atomic force microscopy, transmission electron microscope, and photoluminescence (PL). Attempts were made to grow InAs-QDs using the In-interruption growth technique, in which the In flux was periodically interrupted. QDs grown without using the In-interruption growth technique were grown randomly on all regions. On the other hand, in the case of QDs grown using the In-interruption growth technique, QDs were self-aligned at the boundary between bright and dark regions, the PL intensity was increased and the PL peak position of QDs were red-shifted to 1300 nm. This represent a new technique for growing self-aligned QDs because no extra processing such as electron-beam lithography, V-grooves and surface modification by scanning tunneling microscopy is needed, and aligned QDs can be grown in situ on conventional GaAs substrates.     

Phase separation phenomenon in MOCVD-grown GaInP epitaxial layers

In-rich and Ga-rich GaInP films were intentionally grown on (0 0 1) GaAs substrates by low-pressure MOCVD to investigate the effect of lattice strain on composition. High-resolution X-ray diffraction (HRXRD) measurement showed that a GaInP single layer exhibits a double-diffracted peak phenomenon. Such a double peak represents a composition separation in the grown film, resulting in two absorption cutoff energies in optical absorption analysis. Cross-sectional transmission electron microscopic (TEM) observation confirmed the composition separation in an In-rich GaInP film. Furthermore, the composition separation amount of a Ga-rich GaInP film after substrate removal was found to be ∼0.5%, which reflects the actual effect of lattice strain on composition during growth stage.     

MOVPE growth of GaAs on Ge substrates by inserting a thin low temperature buffer layer

High quality GaAs layers have been grown by low pressure MOVPE on Ge(001) and Ge(001) 9° off oriented in [110] direction by using a thin low temperature (LT) GaAs layer. Investigations of the initial growth step were performed at different V/III ratios and temperatures. To show the good buffer layer quality solar cell structures were grown on off oriented n‐Ge(001) and n‐GaAs(001) substrates. The surface morphology was studied by atomic force microscopy which showed the step‐flow growth mode on 1.2 µm thick GaAs/Ge structures. The crystalline qualities of this structures and the smooth surface morphology were investigated by double crystal X‐ray diffraction (XRD) and atomic force microscopy (AFM). (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Characterization of heterointerfaces in GaAs/MnAs/MnZn-ferrite structures

We have grown GaAs epitaxial films on MnZn-ferrite substrates using MnAs buffer layers and investigated their heterointerfaces with glazing incidence-angle X-ray reflectivity and X-ray photoelectron spectroscopy. It has been found that the heterointerfaces for this structure are quite abrupt and the roughness at the GaAs/MnAs and MnAs/MnZn-ferrite interfaces are 1.1 and 0.2 nm, respectively. We also found that the diffusion of atoms through the GaAs/MnAs interface into the GaAs film is negligible. These results indicate that the MnAs buffer layer for the GaAs/ferrite structure is chemically stable and promising for the application to the future magnetic electronics.     

Growth of InAs on GaAs(1 0 0) by low-pressure metalorganic chemical vapor deposition employing in situ generated arsine radicals

InAs was grown by low-pressure metalorganic chemical vapor deposition on vicinal GaAs(1 0 0) substrates misoriented by 2° toward [0 0 1]. We observed InAs crystal growth, at substrate temperatures down to 300°C, employing in situ plasma-generated arsine radicals as the arsenic source. The in situ generated arsine was produced by placing solid arsenic downstream of a microwave driven hydrogen plasma. Trimethylindium (TMIn) feedstock carried by hydrogen gas was used as the indium source. The Arrhenius plot of InAs growth rate vs. reciprocal substrate temperature displayed an activation energy of 46.1 kcal/mol in the temperature range of 300–350°C. This measured activation energy value is very close to the energy necessary to remove the first methyl radical from the TMIn molecule, which has never been reported in prior InAs growth to the best of authors’ knowledge. The film growth mechanism is discussed. The crystallinity, infrared spectrum, electrical properties and impurity levels of grown InAs are also presented.     

Structural and optical properties of CuGaSe2 layers grown by the hot wall epitaxy method

Copper gallium selenide (CuGaSe₂, CGS) layers were grown by the hot wall epitaxy method. The optimum temperatures of the substrate and source for growth turned out to be 450 and 610 °C, respectively. The CGS layers were epitaxially grown along the 1 1 0 direction and consisted of Ga-rich components indicating the slight stoichiometric deviations. Based on the absorption measurement, the band-gap variation of CGS was well interpreted by the Varshni's equation. The band-gap energies at low temperatures, however, had a higher value than those of other CGS. It suggests that the band-gap increase is influenced by the slightly Ga-rich composition. From the low-temperature photoluminescence experiment, sharp and intensive free- and bound-exciton peaks were observed. By analyzing these emissions, a band diagram of the observed optical transitions was obtained. From the solar cell measurement, an 11.17% efficiency on the n-CdS/p-CGS junction was achieved.     

Shape transition from InAs quantum dash to quantum dot on InP(3 1 1)A

We report on the shape transition from InAs quantum dashes to quantum dots (QDs) on lattice-matched GaInAsP on InP(3 1 1)A substrates. InAs quantum dashes develop during chemical-beam epitaxy of 3.2 monolayers InAs, which transform into round InAs QDs by introducing a growth interruption without arsenic flux after InAs deposition. The shape transition is solely attributed to surface properties, i.e., increase of the surface energy and symmetry under arsenic deficient conditions. The round QD shape is maintained during subsequent GaInAsP overgrowth because the reversed shape transition from dot to dash is kinetically hindered by the decreased ad-atom diffusion under arsenic flux.     

GaN/GaAs(1 0 0) superlattices grown by metalorganic vapor phase epitaxy using dimethylhydrazine precursor

Superlattices of cubic gallium nitride (GaN) and gallium arsenide (GaAs) were grown on GaAs(1 0 0) substrates using metalorganic vapor phase epitaxy (MOVPE) with dimethylhydrazine (DMHy) as nitrogen source. Structures grown at low temperatures with varying layer thicknesses were characterized using high resolution X-ray diffraction and atomic force microscopy. Several growth modes of GaAs on GaN were observed: step-edge, layer-by-layer 2D, and 3D island growth. A two-temperature growth process was found to yield good crystal quality and atomically flat surfaces. The results suggest that MOVPE-grown thin GaN layers may be applicable to novel GaAs heterostructure devices.     

Characterization of InAs quantum dots on lattice-matched InAlGaAs/InP superlattice structures

Self-assembled InAs quantum dots (QDs) in an InAlGaAs matrix, lattice-matched to InP substrate, have been grown by molecular beam epitaxy (MBE). Transmission electron microscopy (TEM), double-crystal X-ray diffraction (DCXRD) and photoluminescence (PL) are used to study their structural and optical properties. In InAs/InAlGaAs/InP system, we propose that when the thickness of InAs layer deposited is small, the random strain distribution of the matrix layer results in the formation of tadpole-shaped QDs with tails towards random directions, while the QDs begin to turn into dome-shaped and then coalesce to form islands with larger size and lower density to release the increasing misfit strain with the continuous deposition of InAs. XRD rocking curves showing the reduced strain with increasing thickness of InAs layer may also support our notion. The results of PL measurements are in well agreement with that of TEM images.     

Incorporation of group III,IV and V elements in 3C–SiC(1 1 1) layers grown by the vapour–liquid–solid mechanism

We report on a comparative investigation of the incorporation of group III, IV and V impurities in 3C–SiC heteroepitaxial layers grown by the vapour–liquid–solid (VLS) mechanism on on-axis α-SiC substrates. To this end, various Si-based melts have been used with addition of Al, Ga, Ge and Sn species. Homoepitaxial α-SiC layers grown using Al-based melts were used for comparison purposed for Al incorporation. Nitrogen incorporation depth profile systematically displays an overshoot at the substrate/epilayer interface for all the layers. Ga and Al incorporations follow the same distribution shape as N whereas this is not the case for the isoelectronic impurities Ge and Sn. This suggests some interaction between Ga/Al and N coming from the high bonding energy between the group III and V elements, which does not exist with Ge and Sn. This is why both incorporate as a cluster. A model of incorporation is proposed taking into account metal-N and metal-C bonding energies together with the solid solubility of the corresponding nitrides.     

Nucleation and ripening of seeded InAs/GaAs quantum dots

We have investigated the nucleation and ripening of pairs of InAs/GaAs quantum dot layers separated by thin (2–20 nm) GaAs spacer layers. Reflection high energy electron diffraction (RHEED) measurements show that the 2D–3D transition in the second layer can occur for less than 1 monolayer deposition of InAs. Immediately after the islanding transition in the second layer chevrons were observed with included angles as low as 20° and this angle was seen to increase continuously to 45±2° as more material was deposited. Atomic force microscopy showed the dot density in both layers to be the same. It is proposed that surface morphology can radically alter processes that determine the nucleation and ripening of the 3D islands.     

Influence of initial growth parameters on the structural and optical properties of GaAs on (001) Si

The structural and optical properties of GaAs on (001) Si substrates were investigated by transmission electron microscopy (TEM) and low-temperature photoluminescence (PL). It was found that the success of the two-step growth technique is controlled by the quality (morphology and defect density) of the low-temperature grown AlGaAs nucleation layer. GaAs epilayers grown on low V/III ratio AlGaAs nucleation layers exhibit improved surface morphologies and structural properties. These results were confirmed by optical measurements where it was shown that the best PL response was obtained from GaAs epilayers in which the initial AlGaAs nucleation layers were deposited at a low V/III ratio.     

Influence of Be on N composition in Be-doped InGaAsN grown by RF plasma-assisted molecular beam epitaxy

Undoped and Be-doped InGaAsN layers were grown on GaAs substrates under the same growth conditions by radio frequency plasma-assisted molecular beam epitaxy. Increased tensile strain (Δa/a=3×10⁻³) was observed for Be-doped InGaAsN layers, compared to undoped InGaAsN layers. The strain is shown to originate from the increase in N composition related to Be incorporation, rather than solely from Be atoms substituting Ga atom sites (Be_Ga). A possible reason is the high Be–N bond strength, which inhibits the loss of N from the growth surface during epitaxial growth, thereby increasing the N composition in the Be-doped InGaAsN layer.     

Growth of high quality ZnSe on closely lattice-matched InGaAs substrates by metal organic chemical vapor deposition

Epitaxial ZnSe layers have been grown by metal organic chemical vapor deposition (MOCVD) on GaAs and InGaAs substrates over the temperature range 400–500°C, using either diisopropyl selenide or diethyl selenide with diethyl zinc. The latter combination leads to improved optical and crystal quality at a growth temperature of 500°C. The narrowest double crystal rocking curve width is 100 arcsec in the lattice-matched case with a 3.5% InAs content in the InGaAs substrate, comparable to films grown by molecular beam epitaxy (MBE). Both n- and p-type dopants have been incorporated to fabricate p/n homojunction structures.     

Preparation of Ge-Si epitaxial alloys by sputtering

Ge-Si alloy layers have been epitaxially grown throughout the whole range of composition onto Ge substrates by the simultaneous getter sputtering from elemental Ge and Si sources. The epitaxial temperature was 550 to 830° C at growth rates of about 1 μm/h, depending on the Si atomic fraction in the range of 0.05 to 0.88. As the Si content in the alloy increases, the crystallinity of the layer decreases: Si-rich alloy layers contained microtwins. Hall measurements of alloy layers without intentional doping indicated p-type conductivity with Hall mobility of 600 cm²/V·sec at carrier concentration of 2 × 10¹⁶ cm^-3 for 25 at% Si in the alloy at room temperature. The observed temperature dependence of the hole mobility is indicative of alloy scattering.     

MOCVD growth of GaN on Si(1 1 1) substrates using an ALD-grown Al2O3 interlayer

GaN thin films have been grown on Si(1 1 1) substrates using an atomic layer deposition (ALD)-grown Al₂O₃ interlayer. This thin Al₂O₃ layer reduces strain in the subsequent GaN layer, leading to lower defect densities and improved material quality compared to GaN thin films grown by the same process on bare Si. XRD ω-scans showed a full width at half maximum (FWHM) of 549 arcsec for GaN grown on bare Si and a FWHM as low as 378 arcsec for GaN grown on Si using the ALD-grown Al₂O₃ interlayer. Raman spectroscopy was used to study the strain in these films in more detail, with the shift of the E₂(high) mode showing a clear dependence of strain on Al₂O₃ interlayer thickness. This dependence of strain on Al₂O₃ thickness was also observed via the redshift of the near bandedge emission in room temperature photoluminescence (RT-PL) spectroscopy. The reduction in strain results in a significant reduction in both crack density and screw dislocation density compared to similar films grown on bare Si. Screw dislocation density of the films grown on Al₂O₃/Si substrates approaches that of typical GaN layers on sapphire. This work shows great promise for the use of oxide interlayers for growth of GaN-based LEDs on Si.